Conventionally, an automatic document feeder arranged in an image forming device may be cited as an example of a sheet feeder that feeds a sheet. Such an automatic document feeder includes a document feed tray, a document conveyance section, and a document discharge tray. When a document (sheet) is placed on the document feed tray, the document is conveyed by the document conveyance section so as to pass a predetermined document reading position. The document from which a document image is read at the document reading position is discharged to the document discharge tray.
A size detection section that detects a size of a document placed on the document feed tray is arranged in the automatic document feeder described above. A size of the document in a width direction thereof is detected by a detection mechanism interlocked with a cursor that restricts a position of the document. Meanwhile, a size of the document in a paper feed direction is detected by a detection sensor arranged at a predetermined position of the document feed tray. The detection sensor irradiates detection light upward and receives detection light reflected by the document. The size of the document in the paper feed direction is detected based on a detection result of the detection sensor. Japanese Unexamined Patent Publication No. H8-301457 discloses a technique including a suction mechanism that suctions a risen document toward a side of a document feed tray in order to reliably detect a size of the document when a plurality of sheets are placed on the document feed tray.